Abstract
In a recent brain imaging study, we demonstrated for the first time that highly symbolic expressive hand gestures bearing emotional significance are selectively processed by the brain. In the present study, we extended these findings by examining whether emotional hand gestures also elicit according electro-cortical processing signatures. Specifically, we wanted to determine whether a well established emotion-related negative ERP component occurring at around 150 ms post-stimulus (EPN) is also observable as a function of the emotionality of passively viewed hand gestures. For this purpose, 16 participants viewed rapidly presented images (335 ms) of one pleasant, one neutral and one unpleasant gesture without inter-stimulus gap.
The subjects also rated the experimental stimuli in terms of perceived valence and arousal. Regarding the behavioral measures, all different gesture categories were differentiated from each other with respect to the valence dimension, whereas only the unpleasant gesture was evaluated as being higher arousing than the others. Consistent with a priori assumptions, the emotional gesture categories were associated with an increased posterior negativity at around 150 ms after stimulus onset. This effect was most pronounced for the unpleasant gesture. These results provide converging evidence for the notion that emotional significance reflected in selective neuronal processing is not limited to evolutionarily important stimulus classes, but that it also may get attached to highly symbolic stimuli during the course of life.
Introduction
Over the last years, affective neuroscience has accumulated a wealth of evidence demonstrating the preferential processing of emotionally salient stimuli by the brain. In imaging studies, it is consistently found that emotionally arousing images selectively activate associative visual cortex, as well as structures of cortical and sub-cortical affective networks (e.g., Bradley et al., 2003; Junghöfer et al., 2005; Junghöfer et al., 2006; Sabatinelli et al., 2004; Sabatinelli et al., 2005). The superior significance of emotional picture materials is also reflected at the behavioral level, as indicated by results from experiments utilizing the emotional stroop, dot probe, visual search, and attentional blink tasks (Anderson, 2005; Mathews & MacLeod, 1994; Mogg &
Bradley, 1999; Öhman et al., 2001a; Öhman et al., 2001b). These findings may be integrated theoretically by considering evolutionary accounts (Cacioppo et al., 1999;
Lang et al., 1997; Öhman et al., 2000a). From this perspective, it is highly beneficial for organisms to quickly identify motivationally important stimuli in the environment as a first step towards optimal behavioral selection under any given situational circumstances. Altogether, this emotion selective processing enhances an organism’s survivability and promotes self-sustaining actions and reproductive success.
To examine such emotion related processing previous research has largely relied on picture materials depicting naturalistic scenes, facial expressions and body language. Despite their widely differing semantic and physical characteristics, pictures from all of those stimulus classes are potent elicitors of selective neuronal processing.
Accordingly, converging evidence demonstrates enhanced neural processing in various structures, most prominently in extrastriate cortex, the amygdala and sub-cortical networks when viewing emotionally salient scenes (e.g., Junghöfer et al., 2005; Junghöfer et al., 2006; Sabatinelli et al., 2004; Sabatinelli et al., 2005), facial expressions (e.g., Pessoa et al., 2002b; Vuilleumier et al., 2001) and body postures (de Gelder et al., 2004; Hadjikhani & de Gelder, 2003), respectively. It has been suggested, that these stimuli have inherent emotional meaning because they are
‘biologically prepared’ (see Bradley et al., 2001; Lang et al., 2000; Öhman, 1986;
Öhman & Mineka, 2001). Accordingly, the evolutionary significance of such environmental cues is believed to promote selective perceptual processing, attention capture and rapid aversive learning (see Öhman & Mineka, 2001). Additionally, the emotional relevance of these stimuli appears to be culturally universal. For instance,
Ekman proposed six basic emotions identifiable from characteristic facial expressions by people from varying cultural origin (Ekman & Oster, 1979). However, it remains to be answered to what degree the known neural processing signatures of emotion are dependent on the evolutionary preparedness of the eliciting environmental stimuli.
Specifically, are some of the underlying processes exclusively engaged by stimuli of high evolutionary relevance whereas others may more strongly be influenced by ontogenetic learning biography? For instance, it is conceivable that the earliest stages of selective emotion processing require stimuli whose emotional significance is phylogenetically hard-wired whereas later sub-stages are possibly more prone to individual learning and subjective experience. It might furthermore be possible that the according differentiation between the processing of evolutionarily significant and symbolic emotional stimuli becomes also apparent in terms of neuronal structures mediating the underlying processes.
In a recent study using functional brain imaging, we addressed this question by presenting emotionally meaningful expressive hand gestures to participants who had to passively view the picture stimuli (Flaisch et al., in preparation). We chose hand gestures because they represent important social cues, informative for reasoning about the motivational and emotional states of interaction partners (see Keltner &
Kring, 1998). Still, as opposed to facial expressions and emotional body language serving the same purpose, their meaning is highly symbolic and conventionalized, being strongly dependent on a shared cultural context between the sender and the receiver of the non-verbal message. We hypothesized beforehand that hand gestures bearing emotional significance would be associated with increased BOLD activity in structures corresponding to those reported in studies using emotional stimuli of high evolutionary importance. Consistent with these a priori assumptions, increased activation of extrastriate and parietal cortical areas was found maximally pronounced while watching insulting gestures. We interpreted these results as evidence for the notion that whereas emotional significance may be biologically prepared for some stimulus classes, it nonetheless may be acquired and get attached to formerly meaningless stimuli during the course of life, in effect tapping the identical neuronal correlates.
Even though emotional hand gestures relate to other affective stimuli in terms of neural activation patterns, it needs to be determined on a more fine-grained
temporal scale whether this applies to the identical underlying cognitive processes. For this purpose, event-related potentials (ERPs) have proven to be especially valuable to disentangle the various sub-processes involved. Moreover, existing research suggests that ERPs are a feasible means to specifically investigate the neural processing signature of meaningful hand gestures. For instance, utilizing iconic meaningful gestures Wu and colleagues (2005) report that these stimuli are not only perceptually but rather semantically encoded by demonstrating an enhanced N400 component due to semantic violations when incongruent meaningful hand gestures were paired with each other. In the domain of emotion research, two distinct components indexing separate emotion sensitive processes could be identified. The first of those components becomes apparent at around 150 ms post-stimulus as a more negative going deflection over posterior leads (Junghöfer et al., 2001; Schupp et al., 2003a; Schupp et al., 2003b;
Schupp et al., 2004b). Relating to its comparably short latency and its topographic distribution, it was termed “early posterior negativity” (EPN; see Schupp et al., in press) and was interpreted as an indicator for the enhanced perceptual encoding of motivationally significant stimuli (Schupp et al., 2003b). In contrast, the second emotion-sensitive evoked potential ( “late positive potential”, LPP) is observed between 400-600 ms post-stimulus over centro-parietal sites and is believed to reflect the sustained processing of affectively salient pictures (also referred to as P3b;
Amrhein et al., 2004; Cuthbert et al., 2000; Keil et al., 2002; Palomba et al., 1997;
Schupp et al., 2000; Schupp et al., 2003b; Schupp et al., 2004b; Schupp et al., 2004c).
Thus, to fully assess the inherent emotional quality of expressive hand gestures it is critical to examine whether they are also processed by the brain in according fashion, i.e. whether they trigger the same processes as conventional emotional pictures.
Based on this consideration, the present study was conducted to answer the question whether highly symbolic emotional hand gestures are subject to rapid emotion discrimination as demonstrated previously using complex scenes (Junghöfer et al., 2001). Specifically, we assessed whether the early electro-cortical indicator of selective emotion processing (that is, the EPN) is sensitive to the emotionality of these picture materials. For this purpose, we measured electro-cortical activity while subjects watched rapidly presented pictures (“rapid serial presentation”, RSVP) of the same hand gestures as were examined in the previous imaging study (Flaisch et al., in preparation). Based on the findings from this prior imaging study, we hypothesized that emotional hand gestures trigger the same emotion-selective processes than
conventional emotional picture stimuli. Specifically, we expected an augmented posterior negativity over occipital sensor sites when watching emotionally meaningful as compared to affectively neutral gestures. Moreover, utilizing high-density EEG measurement enabled us to calculate estimations of underlying dipole source localizations of the observed effects. As shown in studies using emotional naturalistic scenes (Schupp et al., 2006), we expected to localize the source generators of the EPN over temporo- and parieto-occipital areas.
Method
Participants
The study was conducted as a project in an undergraduate experimental lab course. Accordingly, all course participants had to conduct one session as experimenter. The overall sample consisted of 8 male and 8 female participants (N=16) who were naïve with respect to the study’s aims and the explored questions.
Participants were between the age of 20 and 33 years (M=22.6).